I was in London last week (which is outside my country, so I did not use roaming to connect to the internet to be able to use the maps application from Google). In stead , I used Mapdroid to find my way around the center there.

Mapdroid, for those who don't know this program, lets you download maps and than use them without data connection. The only thing you need to do is turn on the GPS.

This works great apart for the fact that it can take between 5 and 10 minutes for the program to mark you location. If it does, it can track you fine , and it keeps tracking you on the map.

But I am just wondering why it takes so long before Mapdroid knows where you are.

Is it a hardware thing (I have a Samsung Galaxy S phone running Froyo).

3 Answers
3

This shows the difference between AGPS and GPS. AGPS (Assisted GPS) uses a small amount of mobile data to establish a rough location fix (about 1km-100m accuracy) based on the cell towers near you. This gives a faster initial location fix than just GPS. Since you had roaming disabled, AGPS wasn't available and getting a location fix on just GPS can take a while, especially on some firmware versions of the Galaxy S.

5-10 minutes seems awfully long. Using a GPS receiver like a Garmin in a car takes maybe a minute at the most. Usually they only take longer if line of sight is poor. I tried Mapdroid in Vancouver and it took quite a while to sync up. It never took me 5 minutes or more though.
–
BobFeb 8 '11 at 13:39

Most GPS devices (including Android) "cheat" by using their previous known location to get faster first fix. If you turned off your GPS device, and then moved to a very far away place, and turn it on there, the GPS have to locate from zero information and it will take about 5 minutes for any GPS device to get a another first fix.
–
Lie RyanFeb 10 '11 at 7:46

I've actually been writing an application that uses GPS, so I can shed some light on this. onik is correct about the AGPS vs GPS. Additionally, though, GPS can be affected by a number of factors which can reduce accuracy or the ability to receive a signal at all:

GPS accuracy is affected by a number
of factors, including satellite
positions, noise in the radio signal,
atmospheric conditions, and natural
barriers to the signal. Noise can
create an error between 1 to 10 meters
and results from static or
interference from something near the
receiver or something on the same
frequency. Objects such a mountains or
buildings between the satellite and
the receiver can also produce error,
sometimes up to 30 meters. The most
accurate determination of position
occurs when the satellite and receiver
have a clear view of each other and no
other objects interfere.

As well with AGPS is DGPS (Differential GPS). (But I do not believe phones have this, only AGPS).

Like the AGPS, the DGPS uses a fixed
GPS location (such as a cell tower) to
send information to the GPS receiver.
DGPS, however, looks at both the
satellite and the fixed location
adjusts for any difference between the
two, and then sends that information
to the receiver. DGPS is particularly
helpful when atmospheric conditions
interfere with reception.

Galaxy S has a notoriously bad GPS. I Changed from S to now S3, and there is a HUGE difference. The S3 is extremely fast to get a GPS lock, and incomparably more precise. So don't expect wonders from a Galaxy S.